Fibrous web and process for the preparation thereof

ABSTRACT

The invention relates to a fiber web and a method for preparing it. The fiber web comprises a filler-containing base web, which is possibly coated with a pigment-containing coating layer. According to the invention, 5-100% of the filler in the base web is made up of cellulose fibrils or lignocellulose fibrils with light-scattering material particles deposited thereon. These coated cellulose fibrils or lignocellulose fibrils constitute at maximum approx. 70% of the weight of the base web. The base web according to the invention contains more than 20% less retention agents than does a web which has the same formation but in which more than 95% of the filler is made up of filler particles known per se.

[0001] The present invention relates to the fiber web according to thepreamble of claim 1.

[0002] A fiber web such as this in general comprises a filler-containingbase web possibly coated with a pigment-containing coating layer.

[0003] The invention also relates to the method according to thepreamble of claim 17 for the preparation of a fiber web.

[0004] The slush in papermaking is made up of cellulose fibers orlignocellulose fibers, fines and fillers. Many of these components, inparticular fillers, are so small in size that they will not adheremechanically to the wire and they have to be bound to larger fibers orto each other by flocculation. For flocculation there are used retentionchemicals which are added to the slush before the forming of the web.Retention chemicals to be mentioned include polymeric products such aspolyethylene imine, low molar mass polyacrylamide and polyamine, as wellas cationic starch, guar or polyacrylamine combined with colloidalsilica, alumina or montmorillonite. The amount of retention chemicals isin general at least 0.5% of the dry matter of the fiber, typicallyapprox. 0.6-1% of the dry matter of the fiber.

[0005] In papermaking, the evenness of the web, i.e. formation, and theremaining of slush components on the wire, i.e. retention, are normallyalways compromises of some degree; when one of them is improved, theother one deteriorates. This is logical, since retention normallyrequires the flocculation of small particles into clusters, whereuponthe evenness (i.e. formation) of paper deteriorates. For this reason itis often necessary to run the paper with a lower than desired retentionin order to produce even paper, which increases the amount of solids inthe circulation waters and generates various problems. Furthermore, theretention systems at present available commercially are very sensitiveto chemical changes in circulation water.

[0006] It is an object of the present invention to eliminate thedisadvantages associated with the state of the art and to provide atotally novel solution for producing a fiber web that has simultaneouslyboth good retention and good formation.

[0007] The invention is based on the observation that the amount ofconventional retention agents can be reduced significantly by replacingat least a portion of the conventional mineral fillers, i.e. pulverousmineral pigments, with a composite pigment containing light-scatteringmaterial particles deposited on the surface of the fines. A fillerproduct of this type is known from FI patent publication 100729. Theproduct comprises strings of pearls made up of calcium carbonateparticles deposited on the fines and of fine fibrils. According to thepatent publication, it is characteristic of this filler of a novel typethat calcium carbonate is deposited on fine fibrils prepared fromcellulose fibers and/or mechanical pulp fibers by beating. The sizedistribution of the fines fraction mainly corresponds to wire screenfraction P100.

[0008] On the basis of the said patent publication, by means of thefiller it is possible to increase the concentration of calcium carbonatein paper, whereby the grammage of the paper can be reduced withoutchanging the “other important” properties of the paper. The fillerretention is good. The results of the publication are based on resultsmeasured from laboratory sheets by standards SCAN-C 26:76 andrespectively SCAN-M 5:76. In the laboratory sheets, cationic starch wasused in an amount of 0.65% and silica in an amount of 0.15% of the massof the fiber.

[0009] In connection with the present invention it has been observedunexpectedly that with the composite fillers described above there isobtained at a high level of formation so high a fiber slush retention inthe paper or board machine that conventional retention agents are notrequired at all, or their amount is considerably lower than inconventional base webs.

[0010] According to the invention, 10-100% of the filler in the base webis made up of cellulose fibrils or lignocellulose fibrils on which thereare deposited light-scattering material particles, and these coatedcellulose or lignocellulose fibrils constitute at maximum approx. 70% ofthe weight of the base web.

[0011] More precisely, the fiber web according to the invention ischaracterized in what is stated in the characterizing part of claim 1.

[0012] The method according to the invention for the preparation of afiber web is for its part characterized in what is stated in thecharacterizing part of claim 17.

[0013] The invention provides considerable advantages. Thus, by means ofthe invention, paper of uniform quality as well as a high level offormation and high retention can be produced; additionally thecirculation waters are substantially cleaner and the retention agentrequirement is reduced. The invention does not require any flocculationof the filler, contrary to the commercially available retention systems;this has a fundamental effect on formation and the optical effect of thefiller.

[0014] Better formation leads to smoother and glossier paper. If thepaper is coated, the cover provided by the coating is better, whichmakes smaller amounts of coating possible. Problems associated with theunevenness of gloss and print quality are reduced.

[0015] The base web does not contain any conventional retention agents,or their amount is over 20% smaller, preferably up to 50% smaller, thanthat of a web that has the same level of formation and containsconventional particulate fillers. The option according to the inventiontherefore decreases the chemicals costs in the preparation of a fiberweb and facilitates the recycling of waters in a paper or board machine.According to the invention it is possible to decrease the amounts ofboth conventionally used nanoparticles and polymers (cf. above), oreither one of them. The said reduction of 20% can thus be calculatedseparately from one of the components or from both components in total.In Example 3 below, the amount of the polymer component has been reducedby slightly less than 30% by weight while the quantity of nanoparticleshas remained unchanged.

[0016] The invention will be described below in greater detail with thehelp of a detailed specification and a few embodiment examples.

[0017]FIG. 1 shows, of the results of Example 2, the web formation as afunction of the total retention, and

[0018]FIG. 2 shows the formation as a function of the wire waterconsistency.

[0019] In the examples below, the filler used contains a fines fractioncomprising fibrils obtained from a chemical pulp. By “chemical pulp” ismeant in this context a pulp that has been treated with digestionchemicals for the delignification of cellulose fibers. According to onepreferred embodiment, the fibrils used in the invention are fibrilsobtained by beating from pulps prepared by the sulfate process and byother alkaline processes. In addition to chemical pulps, the inventionis also suited for fillers produced from fibrils obtained fromchemimechanical and mechanical pulps.

[0020] Typically the average thickness of cellulose or lignocellulosefibrils is smaller than 1 μm. The fibrils are characterized by one orboth of the following criteria:

[0021] a. they correspond to a fraction which passes a 50 mesh screen;

[0022] b. their average thickness is 0.01-10 μm (most suitably atmaximum 5 μm) and their average length is 10-1500 μn.

[0023] The source material for the fibrils, i.e. the fines based oncellulose or other fibers, is fibrillated by beating in a pulp refiner.The desired fraction may, when necessary, be separated using a screen,but the fines need not always be screened. Suitable fibril fractionsinclude wire screen fractions P50-P400. Preferably refiners with groovedblades are used.

[0024] The light-scattering material particles in the filler areinorganic or organic salts which can be formed from their sourcematerials by precipitation in an aqueous medium. Such compounds includecalcium carbonate, calcium oxalate, calcium sulfate, barium sulfate, andmixtures thereof. The material particles are precipitated on the fibrilsin order to produce an aggregate resembling a string-of-pearls. Theamount of the inorganic salt compound is approx. 0.0001-95% by weight,preferably approx. 0.1-90% by weight, most suitably approx. 60-80% byweight, calculated from the amount of filler, and approx. 0.1-60% byweight, preferably approx. 0.5-50% by weight of the paper.

[0025] The invention is discussed below by using the product accordingto FI patent publication 100729 as an example, but it is clear that itis possible to use in the invention any other of the above-mentionedproducts which contain various light-scattering pigments.

[0026] The filler is prepared by depositing the mineral pigment on thesurface of fine fibrils prepared from cellulose fibers and/or mechanicalpulp fibers. For example, the precipitation of calcium carbonate can becarried out by feeding into an aqueous slush of fibrils an aqueouscalcium hydroxide mixture, which possibly contains solid calciumhydroxide, and a compound that contains carbonate ions and is at leastpartly dissolved in water. It is also possible to introduce carbondioxide gas into the aqueous phase, which gas in the presence of calciumhydroxide produces calcium carbonate. There form string-of-pearls-likecalcium carbonate crystal aggregates which are held together by fibrils,i.e. fine strands, and in which the calcium carbonate particles aredeposited onto the fine fibrils and attached to them. The fine fibrilstogether with calcium carbonate form string-of-pearls-like strands,which primarily resemble strings of pearls in a pile. In water (slush)the ratio of the effective volume of the aggregates to the pulp is veryhigh compared with the corresponding ratio of conventional calciumcarbonate used as filler. By “effective volume” is meant the volumerequired by the pigment.

[0027] The diameter of the calcium carbonate particles in the aggregatesis approx. 0.1-5.1 μm, typically approx. 0.2-3 μm. Usually fibrilscorresponding in the main (at least more than 55%) to wire screenfractions P50-P400 are used.

[0028] The paper pulp is slushed in a manner known per se to a suitableconsistency (typically a solids content of approx. 0.1-1%) and is spreadonto the wire. There is added to the fiber slush, most suitably in theheadbox of the paper or board machine, the above-mentioned filler,usually in an amount of approx. 1-100% by weight of the weight of thefibers in the fiber pulp, in other words the amount of filler may be upto equal to the amount of the actual fiber pulp. Usually the disclosedfiller constitutes at least 5% by weight, most suitably 10-100% byweight, of the filler in the base web, and respectively 10-50% by weightof the fiber material in the base web. It is in principle also possibleto prepare a base web in which the fiber material in its entirety ismade up of filler fibrils, and thus in general the present filler mayconstitute 1-100% by weight of the fiber material in the base web.Preferably coated cellulose fibrils or lignocellulose fibrils constituteat maximum approx. 70%, e.g. approx. 10-65%, of the weight of the baseweb, in which case the rest of the web comprises conventional mechanicaland/or chemical pulp used in papermaking and/or conventional fillersand/or other additives.

[0029] A portion (at maximum 95%, usually 90-10% by weight, of the totalamount of filler) of the filler used in the slush may consist ofconventional fillers, such as calcium carbonate (natural orprecipitated), kaolin, talc, hydrogenated aluminum oxides (aluminumtrihydroxides), calcium sulfate, barium sulfate, calcium oxalate, ortitanium dioxide. Preferably, however, at least 80%, especiallypreferably at least 90%, of the precipitated light-scattering pigmentparticles are attached to fibrils.

[0030] By “conventional filler” is meant here a particulate filler whichis pulverous and comprises loose particles, whereas in the presentinvention the filler used is mainly a product in which the particles arenot loose but attached to fibrils. According to one viewpoint of theinvention, the formation and retention of paper are improved by using aportion of the filler in a form bound to fibrils. Thus, according tothis embodiment, the filler used is at least partly for example calciumcarbonate, calcium oxalate, calcium sulfate or barium sulfate, of whicha portion is in a pulverous form and a portion in a form attached tofibrils. In this embodiment, in order to improve formation, a portion(at least approx. 5% by weight, preferably at least 10% by weight, mostsuitably at least 20% by weight) of the pulverous filler is replacedwith a product attached to fibrils.

[0031] In the paper or board machine the fiber pulp is formed into apaper or board web. The fiber web is dried and most suitably coated, andoptionally after-treated by for example calendering.

[0032] The web can be coated with, for example, calcium carbonate,gypsum, aluminum silicate, kaolin, aluminum hydroxide, magnesiumsilicate, talc, titanium dioxide, barium sulfate, zinc oxide, syntheticpigment, or mixtures thereof.

[0033] The light-scattering material particles of the filler in the webare present in the fiber web in a substantially unflocculated form. Thismeans that the formation of the web is quite good. Thus, at formationlevel 10 the web contains conventional retention agents, such ascationic starch and/or silica, in total at maximum 0.40% by weight ofthe mass of the fiber. According to an especially preferred embodimentthe web is substantially or entirely free of ionic retention agents.“Conventional retention agents” in particular include those usedtogether with conventional fillers.

[0034] With the help of the invention it is possible to produce coatedand optionally also calendered cellulose-containing material webs havingexcellent printing properties, high smoothness, as well as high opacityand whiteness. By “cellulose-containing material” is meant heregenerally paper or board or a corresponding cellulose-containingmaterial derived from a lignocellulose-containing raw material, inparticular wood or annual or perennial plants. The said material may bewood-containing or woodfree, and it may be prepared from mechanical,semimechanical (chemimechanical) or chemical pulp. The chemical pulp andthe mechanical pulp may be bleached or unbleached. The material may alsocontain recycled fibers, in particular recycled paper or recycled board.The grammage of the material web varies typically within the range35-500 g/m², in particular it is approx. 50-450 g/m².

[0035] In general the grammage of base paper is 20-250 g/m², preferably30-80 g/m². By coating a base paper of this type, having a grammage ofapprox. 50-70 g/m², with 10-20 g of coating/m²/side and by calenderingthe paper, there is obtained a product having a grammage of 70-110 g/m²,whiteness of at least 90% and opacity of at least 90%. An especiallypreferred product is a coated offset paper in which high gloss and highopacity and bulk are combined. The invention is also suited for theproduction of coated fine papers, possibly also containing mechanicalpulp, as well as writing and printing papers.

[0036] The following non-restrictive examples illustrate the invention.The measuring results indicated in the examples for the properties ofthe paper were determined by the following standard methods:

[0037] Surface roughness: SCAN-P76:95

[0038] Porosity: SCAN-P60

[0039] Air resistance: SCAN-M8, P19

EXAMPLE 1

[0040] Preparation of filler

[0041] Beating of Chemical Pulp

[0042] Birch sulfate pulp was beaten in Valmet's JC-01 refiner in orderto produce a pulp suitable for the preparation of filler. Theconsistency during the beating was approx. 4%, its total energyconsumption 343 kWh/t and its specific edge load 0.5 J/m.

[0043] The properties of the product are shown in Table 1. TABLE 1 Fiberproperties before and after beating Before After beating beating Fiberlength (length), mm 0.86 0.58 Fiber length (weight), mm 1.00 0.77 SR° 1686

[0044] Carbonation of Fiber Pulp

[0045] Carbonation was carried out in tap water in accordance with whatis disclosed in FI patent 100729. An aqueous slurry having a dry mattercontent of 2.22% was obtained. The concentration of CaCO₃ in the finalproduct was 69.7% and its specific surface area was 10.6 m²/g. The PCCparticle size was of an order of magnitude corresponding to that inExample 1 of the FI patent.

EXAMPLE 2

[0046] The product described in Example 1 was used as filler in coatedfine papers. The following table depicts the results of a fine papertest run carried out in Stockholm with STFI's pilot machine (FEX): TABLE2 Results of fine paper runs Filler Wire water concentration,consistency, Total % g/l retention, % Formation, % PCC 18.8 2.03 72.910.6 PCC 21.0 2.68 66.9 11.1 SuperFill 17.8 0.48 91.1 10.4 SuperFill22.4 0.67 88.3 9.5

[0047] The PCC used in the tests was Albacar LO product supplied bySpecialty Minerals.

[0048] The advantage of the invention is well visible when theproperties are examined simultaneously in the same coordinate system inthe manner shown in accompanying FIGS. 1 and 2.

[0049] The method is provided special value by the fact that theabove-mentioned combinations of properties are achieved simultaneouslywith better optical properties and strengths.

EXAMPLE 3

[0050] Mill Trial

[0051] Preparation of Filler-Containing Base Paper

[0052] For purposes of coating, base paper having a grammage of 56 g/m²was prepared in mill conditions. The slush consisted of a mixture ofbirch pulp (74%) and pine pulp (24%). After beating, the SR number ofthe pine pulp was 32-34° and that of the birch pulp was 22-25°. The SRnumber of the pulp in the head box was 35-40°.

[0053] The wire section of the paper machine was Valmet's hybride wire(Sym-former), and the wet press end comprised Valmet's Sym-Press IIhaving a triple press and a conventional drying section.

[0054] Three different fillers were used in the base paper, namelyFinntalc F 15 SL (talc of Mondo Minerals), Albacar HO (PCC of SpecialtyMinerals), and the composite filler described in Example 1, of which thename “SuperFill” is also used hereinafter. Talc was used as filler inamounts of 10% and 15%, and PCC and SuperFill in amounts of 10%, 15% and20%.

[0055] The retention chemicals used were nanoparticles and cationicstarch (Compozil Plus: EKA NP 780 nanoparticle and EKA PL 1510 C-Pam,supplier: EKA Chemicals). For talc and PCC the doses were as follows:nanoparticle 280 g/t, polymer 70 g/t, and those for SuperFill were:nanoparticle 280 g/t, polymer 50 g/t. The amounts of cationic starch andresin size used were 8 kg/t and 5.2 kg/t. Alum was dosed for talc in anamount of 13 kg/t and for PCC and SuperFill in an amount of 19 kg/t.

[0056] Results

[0057] The different fillers showed no notable differences incirculation water conductivity, COD, pH, cation requirement, ordissolved calcium concentrations. Filler retention was best withSuperFill (40-50%) even though the dose of retention polymers wasconsiderably smaller than with other fillers. The filler retention withtalc and PCC was only 30-40%. The good retention of SuperFillsignificantly reduces the consistency and turbidity of circulation wateras compared with the other fillers. No differences were observable inbeta formation among the different filler and filler concentrations.

1. A fiber web comprising a filler-containing base web possibly coatedwith a pigment-containing coating layer, characterized in that 5-100% ofthe filler in the base web is made up of cellulose fibrils orlignocellulose fibrils with light-scattering material particlesdeposited thereon, and the coated cellulose fibrils or lignocellulosefibrils constitute at maximum approx. 70% of the weight of the base web,the base web containing more than 20% less of retention agents than aweb that has the same formation but wherein more than 95% of the filleris made up of filler particles known per se.
 2. The fiber web accordingto claim 1, characterized in that the filler comprises cellulose fibrilsor lignocellulose fibrils prepared from vegetable fibers by beating andscreening and having an average thickness smaller than 10 μm, preferablyat maximum 5 μm.
 3. The fiber web according to claim 2, characterized inthat the light-scattering material particles are deposited on fibrilswhich correspond to a fraction passing a 50 mesh screen and/or theaverage thickness of which is 0.01-5 μm and average length 10-1500 μm.4. The fiber web according to any of claims 1-3, characterized in thatthe light-scattering material particles are inorganic or organic saltswhich can be formed from their source materials by precipitation in anaqueous medium.
 5. The fiber web according to claim 4, characterized inthat the light-scattering material particles are calcium carbonate,calcium oxalate, calcium sulfate, barium sulfate or a mixture thereof.6. The fiber web according to any of claims 1-5, characterized in thatthe material particles are deposited on fibrils in order to produce astring-of-pearls-like filler.
 7. The fiber web according to claim 6,characterized in that the amount of the calcium compound in proportionto the fiber amount is 0.0001-90% by weight.
 8. The fiber web accordingto any of the preceding claims, characterized in that it is coated withcalcium carbonate, gypsum, aluminum silicate, kaolin, aluminumhydroxide, magnesium silicate, talc, titanium dioxide, barium sulfate,zinc oxide, synthetic pigment, or a mixture thereof.
 9. The fiber webaccording to any of the preceding claims, characterized in that thefiller comprising cellulose fibrils or lignocellulose fibrils withlight-scattering material particles deposited thereon constitutes10-100% by weight of the filler in the base web.
 10. The fiber webaccording to any of the preceding claims, characterized in that thefiller comprising cellulose fibrils or lignocellulose fibrils withlight-scattering material particles deposited thereon constitutes 10-50%by weight of the fiber material in the base web.
 11. The fiber webaccording to any of the preceding claims, characterized in that thelight-scattering material particles in the filler of the web are presentin a substantially unflocculated form in the fiber web.
 12. The fiberweb according to any of the preceding claims, characterized in that at aformation level of 10 it contains retention agents in an amount of0-0.40% by weight of the mass of the fiber.
 13. The fiber web accordingto claim 12, characterized in that it is substantially free of ionicretention agents.
 14. The fiber web according to any of the precedingclaims, characterized in that it comprises coated fine paper or printingor writing paper.
 15. The fiber web according to any of the precedingclaims, characterized in that 5-90% of the filler in the base web ismade up of cellulose fibers or lignocellulose fibers withlight-scattering material particles deposited thereon.
 16. The fiber webaccording to claim 15, characterized in that the balance is made up ofconventional pulverous fillers such as calcium carbonate (natural orprecipitated), kaolin, talc, hydrogenated aluminum oxides (aluminumtrihydroxides), calcium sulfate, barium sulfate, calcium oxalate ortitanium dioxide.
 17. A method for preparing a fiber web, according towhich method there is prepared from a fiber material a slush from whicha fiber web is formed in a paper or board machine, characterized in thatto the slush of fiber material there is added as a filler a productcomprising cellulose fibrils or lignocellulose fibrils withlight-scattering material particles deposited thereon, this productconstituting 5-100% of the filler and its amount being at maximum 70% ofthe mass of the fiber material, and retention agents are dosed into theslush in an amount of 0-0.40% by weight of the mass of the fibermaterial.
 18. The method according to claim 17, characterized in that afiller is used which comprises cellulose fibrils or lignocellulosefibrils prepared from vegetable fibers by beating and screening andhaving an average thickness smaller than 10 μm, preferably at maximum 5μm.
 19. The method according to claim 18, characterized in that a filleris used wherein the light-scattering material particles are deposited onfibrils which correspond to a fraction passing a 50 mesh screen and/orthe average thickness of which is 0.1-5 μm and average length 10-1500μm.
 20. The method according to any of claims 17-19, characterized inthat a filler is used wherein at minimum 90% of precipitatedlight-scattering pigment particles are attached to fibrils.
 21. Themethod according to any of claims 17-20, characterized in that the baseweb is coated in the paper machine with a pigment-containing coatingmix.